So glad you asked.

Soda firing is a type of stoneware firing wherein soda ash and baking soda are introduced to the flame when the kiln is nearly at its hottest. The heat of the firing splits the sodium carbonate and sodium bicarbonate (soda ash and baking soda, respectively) into their component elements, sodium and carbon. The flame carries this vapor into the firing chamber and onto the pots. The sodium interacts with the silica and calcium carbonate in the ware (as well any slips or glazes that have been applied to the ware) to form a layer of glass. It's fired to a very high temperature - between 2200 and 2400 degrees fahrenheit - so it's very durable.

The flame deposits the sodium more heavily wherever it strikes the ware directly, and more lightly on the lee sides of pots, making for a delightful irregularity. This can be observed whenever one side of a pot is very glassy and shiny (as in the butterdish above), sometimes with the design softened or melted away entirely. In fact this is the great appeal of soda firing for me: it allows the flame to collaborate in the final result.

For me, the soda vapor glazing does not replace but instead supplements applied glaze. I choose colors and carefully apply designs, and wait with delighted anticipation to see what the flame does to them. Soda firing is not for control freaks! But if you love spontaneity and uniqueness and surprises, if you can see beauty in irregularity, it might be for you.